ESTIMATION OF PROTEIN DEGRADABILITY IN THE RUMEN FROM INCUBATION MEASUREMENTS WEIGHTED ACCORDING TO RATE OF PASSAGE

A method is proposed for estimating the percentage of dietary protein that is degraded by microbial action in the rumen when protein supplement is added to a specified ration. The potential degradability, p,is measured by incubating the supplement in artificial-fibre bags in the rumen and is related to incubation time, t, by the equation p= a + b(1 - e-ct). The rate constant k, measuring the passage of the supplement from the rumen to the abomasum, is obtained in a separate experiment in which the supplement is combined with a chromium marker which renders it completely indigestible. The effective percentage degradation, p, of the supplement, allowing for rate of passage, is shown to be p = a + [6c/(c + k)] (1 - e-(c+k)t) by time, t, after feeding. As t increases, this tends to the asymptotic value a + bc/(c + k), which therefore provides an estimate of the degradability of the protein supplement under the specified feeding conditions.The method is illustrated by results obtained with soyabean meal fed as a supplement to a dried-grass diet for sheep. The incubation measurements showed that 89 of the soyabean protein disappeared within 24 h and indicated that it was all ultimately degradable with this diet.When the dried grass was given at a restricted level of feeding the allowance for time of retention in the rumen reduced the estimate of final degradability to 71 (69 within 24 h). With ad libitum feeding there was a faster rate of passage and the final degradability was estimated to be 66 (65 within 24 h). © 1979, Cambridge University Press. All rights reserved.